New mechanisms and function of lumbosacral orthoses (LSOs) in the management of low back pain (LBP) are explored in this application by testing four hypotheses. These hypotheses are based on the rationale of LSO-enhanced spine stability, which has not been considered in this context before. The LSO provides additional stability to the lumbar spine and, therefore, patients with LBP will (i) exhibit less trunk antagonistic muscle co-contraction when wearing the LSO as compared to the "No LSO" condition, (ii) respond with a smaller number of muscles and longer latency to sudden trunk loading when wearing the LSO as compared to the "No LSO" condition, (iii) walk with gait closer resembling a normal pattern when wearing the LSO as compared to walking without the LSO, and (iv) exhibit enhanced proprioception in the lumbar spine when wearing the LSO as compared to the "No LSO" condition. Twenty LBP subjects for the experimental group and 20 control LBP subjects will be recruited from the Yale Spine Service clinics to test each hypothesis. The subjects in the experimental group will wear the LSO for 4-6 weeks. All subjects will be tested every two weeks with and without the LSO in 3 separate experiments to address each hypothesis. These experiments will consist of (1) the EMG Experiment to address hypotheses (i) and (ii), (2) the Gait Experiment to address hypothesis (iii), and (3) the Proprioception Experiment to address hypothesis (iv). New research developments will allow us to overcome several methodological and conceptual difficulties to make the proposed research now feasible. Therefore, the application of the following features will distinguish the proposed research from past studies: (i) the concept and quantification of spine stability, (ii) an experimental design tuned to detect very small LSO effects in EMG activity, (iii) a novel, submaximal exertion procedure for normalizing EMG data for patients with LBP, who are reluctant or unable to exert maximum effort, and (iv) a long-term (4-6 weeks) study, which may be necessary for acclimatization to the LSO. The results from these studies could be used in formulating rational guidelines for the prescription of LSOs as well as for improving their designs. In addition, data generated from the proposed research will also carry-over into advancing our understanding of the mechanisms and function of ergonomic lumbar supports.